[Electronic M]

On the vertical circuit make sure the tube is good, the resistors are in tolerance, the pots are clean/not open, and check voltages on the vertical tubes.

If that fails connect the yoke or primary of the vertical output transformer to a variac (make sure it is disconnected from the set before connecting to the variac) and slowly turn it up while watching for more deflection. If you don't get more deflection from that then your yoke or vert transformer is bad (which one can be determined by the connection point).

To add to Eric's point if the H osc. is out of tune it can cause poor HV regulation and even cause the HV to drop out. If you have a scope, connect it to a video source (like a DVD player), and measure the horizontal period by looking for the synch pulse, then with the horizontal settings of the scope unchanged connect the scope to the set's H osc. and tune the osc till the width of the osc wave form is as close as you can get it to the one you measured on the composite video....I've done this on sets with pure unobtanium flys before plugging the output tube in as a safety precaution, and it has worked.